Modellierung von Ladungs- und Exzitondynamik in amorphen organischen Halbleitern = Modeling of charge and exciton dynamics in amorphous organic semiconductors

Organische Halbleiter finden Anwendungen in vielen Technolgien, wie organischen Leuchtdioden, organischen Solarzellen oder organischen Transistoren. Die Effizienz der Bauteile wird hierbei maßgeblich von den elektronischen Transporteigenschaften der zugrundeliegenden organischen Halbleiter bestimmt, deren oft amorphe Struktur zu Ladungsperkolationseffekten auf einer Größenordnung von 100nm führen kann. In dieser Arbeit werden Methoden vorgestellt, die es ermöglichen mesoskopischen Ladungstransport in amorphen organischen Halbleitern, auf der Basis zugrundeliegender quantenchemischer Rechnungen, effizient zu simulieren. Dabei werden Methoden zur effizienten Auswertung der Coulomb-Wechselwirkung zwischen Ladungsträgern in kinetischen Monte-Carlo Verfahren (KMC), sowie Algorithmen zur Beschleuningung von Vielteilchen-KMC-Verfahren vorgestellt. Die Methoden werden gegen den prototypischen organischen Halbleiter alpha-NPD validiert. Es wird gezeigt, dass Ladungstransport in gemischten Emissions-Transport Materialien, wie sie in organischen Leuchtdioden Einsatz finden, durch Hüpftransport zwischen Emittermolekülen stattfindet, indem räumliche Distanzen zwischen End- und Anfangszustand durch virtuelle Zustände auf Transportmolekülen überbrückt werden. Desweitern wird gezeigt, dass Ladungsträgerinjektion an metallisch-organischen Grenzflächen bei hohen elektrischen Feldern durch virtuelle Übergangszustände auf organischen Molekülen in der Nähe der Grenzfläche um mehrere Größenordnungen verstärkt wird

In silico studies of OLED device architectures regarding their efficiency

Simulations have become increasingly important to understand and design organic optoelectronic devices, such as organic light emitting diodes (OLEDs) and to optimize their performance by selecting appropriate materials and layer arrangements. To achieve accurate device simulations, it is crucial to consider the interplay between material properties, device architecture, and operating conditions and to incorporate physical processes such as charge injection, transport, recombination, and exciton decay. Simulations can provide insights into device bottlenecks and streamline optimization cycles, eliminating the need for physical prototyping and rationalizing OLED design. In this study, we investigated three heuristic OLED architectures with a 3D kinetic Monte Carlo (kMC) model and compared their quantum efficiency at different operation voltages. Our investigation focused on examining the effects of various layer arrangements on charge and exciton dynamics in OLED devices and establishing design principles for achieving high efficiency, which are consistent with experimental observations. Notably, we find that increasing the thickness of the emissive layer (EML) led to higher luminance efficiency, and that an emitter concentration of approximately 5% results in optimal performance. By using this model, it is possible to rapidly study the influence of many device parameters and explore a broad range of parameter and architecture space within a reasonable time-frame

Systematic kMC Study of Doped Hole Injection Layers in Organic Electronics

Organic light emitting diodes (OLED) play an important role in commercial displays and are promising candidates for energy-efficient lighting applications. Although they have been continuously developed since their discovery in 1987, some unresolved challenges remain. The performance of OLEDs is determined by a multifaceted interplay of materials and device architectures. A commonly used technique to overcome the charge injection barrier from the electrodes to the organic layers, are doped injection layers. The optimization of doped injection layers is critical for high-efficiency OLED devices, but has been driven mainly by chemical intuition and experimental experience, slowing down the progress in this field. Therefore, computer-aided methods for material and device modeling are promising tools to accelerate the device development process. In this work, we studied the effect of doped hole injection layers on the injection barrier in dependence on material and layer properties by using a parametric kinetic Monte Carlo model. We were able to quantitatively elucidate the influence of doping concentration, material properties, and layer thickness on the injection barrier and device conductivity, leading to the conclusion that our kMC model is suitable for virtual device design

De Novo Calculation of the Charge Carrier Mobility in Amorphous Small Molecule Organic Semiconductors

Organic semiconductors (OSC) are key components in applications such as organic photovoltaics, organic sensors, transistors and organic light emitting diodes (OLED). OSC devices, especially OLEDs, often consist of multiple layers comprising one or more species of organic molecules. The unique properties of each molecular species and their interaction determine charge transport in OSCs—a key factor for device performance. The small charge carrier mobility of OSCs compared to inorganic semiconductors remains a major limitation of OSC device performance. Virtual design can support experimental R&D towards accelerated R&D of OSC compounds with improved charge transport. Here we benchmark a de novo multiscale workflow to compute the charge carrier mobility solely on the basis of the molecular structure: We generate virtual models of OSC thin films with atomistic resolution, compute the electronic structure of molecules in the thin films using a quantum embedding procedure and simulate charge transport with kinetic Monte-Carlo protocol. We show that for 15 common amorphous OSC the computed zero-field and field-dependent mobility are in good agreement with experimental data, proving this approach to be an effective virtual design tool for OSC materials and devices

Multiscale Simulation of Photoluminescence Quenching in Phosphorescent OLED Materials

Bimolecular exciton-quenching processes such as triplet–triplet annihilation (TTA) and triplet–polaron quenching play a central role in phosphorescent organic light-emitting diode (PhOLED) device performance and are, therefore, an essential component in computational models. However, the experiments necessary to determine microscopic parameters underlying such processes are complex and the interpretation of their results is not straightforward. Here, a multiscale simulation protocol to treat TTA is presented, in which microscopic parameters are computed with ab initio electronic structure methods. With this protocol, virtual photoluminescence experiments are performed on a prototypical PhOLED emission material consisting of 93 wt% of 4,4ʹ,4ʺ-tris(N-carbazolyl)triphenylamine and 7 wt% of the green phosphorescent dye fac-tris(2-phenylpyridine)iridium. A phenomenological TTA quenching rate of 8.5 × 10$^{-12}$ cm$^{3}$ s$^{-1}$, independent of illumination intensity, is obtained. This value is comparable to experimental results in the low-intensity limit but differs from experimental rates at higher intensities. This discrepancy is attributed to the difficulties in accounting for fast bimolecular quenching during exciton generation in the interpretation of experimental data. This protocol may aid in the experimental determination of TTA rates, as well as provide an order-of-magnitude estimate for device models containing materials for which no experimental data are available